<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <button id="btn">点我</button>
  </body>
  <script>
    // 不同源就会跨域
    let btn = document.getElementById('btn');
    btn.addEventListener('click', () => {
      let xhr = new XMLHttpRequest();
      xhr.open('get', 'http://localhost:3000/getAjax');
      xhr.onload = () => {
        console.log(xhr.responseText);
      };
      xhr.send();
    });
  </script>
</html>
